Prospecting



Jan. 5, 1954 R. J. sTRAwINsKl 2,665,237

PRosPEcTrNG y y Filed May a, 195o 4 sheets-sheet' 1 COM/D 575D PHASE QQ aff/wz. or Tanz, 6,45 gygy/mfp) M PAY/wo .z3 JST Aw//vs/f/ T. BY M@ Jan. 5, 1954 R. J. sTRAwlNsKl PROSPECTING 4 sheets-sheet s Filed May 8, 1950 @Trim TTORNE'YS Q b, mw

R. J. STRAWINSKI PROSPECTING Jan. 5, 1954 4y Sheets-Sheet 4 Filed May 8, 1950 IN V EN TOR. PA y/Wo/v JST A W//l/f/f/ BY A TTOKZYEYH Patented Jan. 5, 1954 PROSPECTING Raymond J. Strawinski, Long Beach, Calif., as-

signor to Texaco Development Corporation, New York, N. Y., a .corporation of Delaware Application May 8 1950, Serial No. 160,634

6 Claims. (Cl. 195--103.'5`)

This invention relates to prospecting for subterranean deposits such as petroleum, gas, and other deposits in the surface of the earth.

Many methods have been proposed in prospecting for petroleum and gas in subterranean deposits but none to date have proven successful or capable of producing reproducible data. One method has been based on the theory that gases, hydrocarbon in character, in subterranean deposits gradually escape and diffuse upwardly into the surface soil. Thus, the detection and measurement of the gas content or related characteristics of such soils .may serve as an indication of the presence of petroleum or gas deposits under the surface.

In practically all such methods, it is proposed to gather soil samples and thereafter subject them to some type of processing to produce Some type of qualitative indication as to the presence or absence of hydrocarbons. In one method, it is proposed to determine the presence of products of hydrocarbon-consuming bacteria and hydrocarbons as by colorimetric methods. Another `method involves a study of the oxidation-reduction potential of soil samples. In another method, uniform soil samples are prepared and planted throughout an area of the earths surface. The samples are inoculated with hydrocarbon-consuming bacteria and examined'after a period of propagation to determine the extent of 4growth, of the bacteria.

In another method, soil samples are placed in a sealed chamber with a gas mixture of ethane, propane or butane, and air or oxygen. The pressure changes due to apparent consumption of the hydrocarbon gas are noted and the residual gas in the chamber may be analyzed. Purportedly, the information obtained thereby is of value in determining the presence or absence of petroleum.

So far as is known, none of the above methods have proven successful. This is believed to be due to the methods of sampling as well as the methods of analyzing the soil samples which are such as to disregard a number of variable factors effecting the results as hereinafter explained.

ne object of the present invention is to provide a novel method of prospecting wherein soil samples from a selected section of the earths surface can be collected and yprocessed to determine with reasonable assurance the presence or absence of petroleum or gas deposit indications.

Another object of the invention is to provide a novel method of soil sampling wherein samples of uniform character leading to accurate and reproducible determinations are secured.

Still another object of the invention is the provision of novel methods of analyses on both qualitative and quantitative bases of such soil samples and their abilities to react to selected gas mixtures.

Further objects and advantages of the invention will appear from the following description and claims taken in connection with the attached drawings wherein:

Figs. 1 to 3, inclusive, illustrate reactors in various stages of the analytical procedure.

Fig. 4 ,is a graph illustrating the incubation cycle of a typical sample.

Fig. 5 is a graph illustrating a preferred method of analyzing the results prior to correlation with the area being prospected.

Fig. 6 illustrates the results of a survey conducted over a selected area of the earths surface.

Fig. 7 illustrates the results of a second and more detailed survey over the same area as that shown in Fig. 5.

In brief, the present invention is directed to a method of prospecting by the collection and processing of soil samples wherein accurate representative samples are collected and processed under substantially standard conditions with all variables controlled, and the growth of the hydrocarbon-consuming microorganisms or other microorganisms, the presence of which it is desired to determine, is under optimum conditions and, therefore, encouraged so that even small numbers thereof that would otherwise be missed can be determined. The relative determination of such microorganisms is accomplished by determination of the amount of consumption of a gas mixture thereby, the rate of such consumption or reaction, and changes in the composition ofthe gas.

.More specifically, the invention contemplates improved methods of soil sampling, preferably by Way of a wet mix, and improved methods of analysis wherein the Ytime ordinarily required for such analyses can be materially reduced.

The method of the present invention can be readily Adistinguished from the prior art including Patent No. 2,349,472 to Taggart issued May 23, 1944. In the Taggart patent, individual soil samples with their varying moisture content and varying nutrient content, assuming some nutrient is present, therein are sealed with a hydrocarbon gas mixed with air or oxygen. The sample is permitted'to stand for a period usually in excess ofabout two hours and preferably from five to fifteen hours and changes in pressure determined periodically by a manometer. Additionally, the residualgas inthe sample may be analyzed.

This method has its defects in that there are a number of uncontrolled variables including the amount of moisture and the amount of nutrient in each sample. One sample may be from a wet area and one from a dry area. Naturally, the moisture in the wet sample will tend to encourage the growth of microorganisms end the consumption of gas. Similar variations may be found in the nutrient content. Such variables are important factors and may vary to an extent that a dry sample containing little nutrient and a large number of microorganisms may consume only a small amount of gas while a wet sample containing a relatively large amount of nutrient and only a few microorganisms may consume a large quantity of gas. Accordingly, the nal results may be exactly contrary to the facts.

In the present invention such variables are eliminated by the use of a relatively large amount of nutrient which amount compared to any moisture content of the sample and any nutrient content is so large that any variations in moisture or nutrient content are substantially ironed out and each sample virtually standardized. Furthermore, the reservoir of nutrient medium, as explained hereinafter, acts to dilute the reacting medium constantly. This keeps the reaction products in a dilute state so that those which are toxic do not harm the growing microorganisms. in pH is prevented. Thus, the only variable factor is the number of microorganisms which vis admittedly variable.

En addition, the use of the nutrient insures that any microorganisms present will grow under optimum conditions and that a maximum growth will be obtained. This is important where only smell numbers of microorganisms such as bacteria are present in the samples. Without such optimum conditions, the resultant growth may be substantially nil and the survey of no value.

A further defect in prior methods such as that disclosed in the above-mentioned Taggart patent, is that no effort is made to preserve the soil samples in their originel condition until they can be tested. Actual tests as discussed hereinafter have shown that such soil samples can vary widely in gas-consuming characteristics between the time they are taken from the soil and the time they are tested unless they are preserved under selected and predetermined conditions.

The sampling procedure has been found to be important, it being necessary to obtain samples which are truly representative of the areas into which the ground under observation has been divided.

Samples may be secured from different depths below the surface, depending to some extent upon the season of the year and the moisture content of the soil. In any case, it is desirable to get sufliciently far beneath the surface to avoid surface variables such as farming, etc. that may change from day to day. It is preferred to secure samples at depths below six inches, preferably at depths of 24" or more, since under some circumstrnces the top 6" layer may be substantially sterile.

It is believed that the depth is governed principally by the limit to which the necessary quantity of O2 can penetrate. In dry soil or in dry weather, Oz will penetrate further than in Wet soil or in wet weather. Probably, the optimum depth is that at which the hydrocarbon such as methane, if present, and the O2 are in optimum balance as regards propagation. oi the. mCOOl* Additionrlly, any material change ganisms. It is desirable that in the case of any one area being prospected, the sampling depth be uniform.

In sampling, several procedures have been found satisfactory. In one such procedure, a socalled test station is set up and a line extended therethrough to a distance about 10 to 35', usually about 25', in each direction. Samples of approximately one quart of soil are taken at the center of the line and at each end thereof. Each sample is then screened by a U. S. standard sieve No. 18 to secure one quart of soil. One third pint of each sample is withdrawn and combined to provide a composite sample of one pint which is considered to come from the test station. It has been found expedient to space such test stations about 250 feet apart.

Investigations conducted. thus far indicate that a minimum of three equal distance soil samples from a 50 ft. line is desirable for a soil sample that is truly representative of the average rate of reaction for that particular soil.

For example, in extracting the samples from the earth, holes from l() to 14 inches deep are dug at three sites along a 50 foot line, one at the center of the station and one 25 feet from each site in the direction of the sampling line. In extracting the soil sample, care should be taken to insure that the side walls of the holes are clear of surface dirt and dirt from boring and that the sample is taken from the side wall, thus excluding the possibility of picking up dirt from the boring operation which will collect in the bottom of the hole.

Another method involves the setting up of square areas ft. on a side in which case samples are taken from the corners of the square and from the center, the samples being screened and aliquot portions taken therefrom to obtain the desired composite sample.

Since sampling is a eld operation and the remaining procedure is best handled in the laboratory, the samples are suitably packaged and maintained at a temperature at which the bacterial content is maintained substantially static until they can be further processed. A temperature of about 42 F. has been found suitable for this purpose. Under such reduced temperature, it has been found that the samples can be stored for as long as four to seven days without any appreciable change in gas consuming activity.

Without such preservation and with storage at ambient temperatures, it has been found that such samples are materially accelerated in activity. In some cases, the change in rate of reaction has been as high as 17 In other cases, it has been found that on standing only one day, the time necessary for the sample to complete its activity has been reduced by two and one-half days, thus indicating a much higher degree of activity than the sample originally possessed. Thus, the rate of reaction which is desirable in comparing the activity of the several samples can be rendered meaningless.

The nutrient mediums or media used can be varied widely as is well known to the bacteriologist. Two such media which gave excellent results in the prospecting herein described were prepared as follows:

MediumI Grams NH4NO3 1.0 MgSO4 0.1 K2HPO4 0.5 CaSO4 0.1

Above ingredients diluted` toV 10Go `mi.A with distilledfwater,v proximate pH- 7.5.

Medium II Grams NHiCl 1.0 K2HPO4 0.5 MgNH4PO4 0.5- CaSOu 01.1

Above ingredients diluted to 1000 ml.. with distilled water, proximate pH 7.5.

In preparing for the gas contact. and determinationV of the activities of the samples, the different soilv samples are mixed with themedium as in4 a. Waring` blendor or like mixer, and aliquots withdrawn by siphoning while agitation is continued. More specifically, the soil composites removed from cold storage are each weighed and the weight recorded. Those composites weighing in excessof the average weight of all, compositesV are reduced to the average weight by pouring the composite onto paper, mixingand removing the excess soil. Each com posite in an amount of about 350 to 400 grams is then placed ina Waring blender or similar mixer, the medium added inY an amount of about *00v ml. and the mixture stirred vigorously for about one minute'. The speed of the mixer is then reduced slightly by meansA of a variable transformer to a rate; above that at whichl layering or any minimal separation will occur and the pH checked. A pil-l' in the range of 6 to 8, preferably about 7.5, is preferred: and the mix* ture so adjusted if necessary.

A Siphon tube is inserted approximately 2 below the surface of thesoilfmedium mixture and vacuum appliedy to the Siphon tube to start the siphoningy action. When, the iiow has begun, the siphon tube is clamped shut, removed from the vacuum liner and connected to a reactor as later described; The siphon tube is opened, pem'litting` about 50 ml. of' the mixture to flow into thef reactor. if the soil-medium mixture` isin a jelliedY phase Oris too viscous to flow.A through the siphon` tube, it may be removed with a pipette.

As an alternative method, although not preferred, the soil may be dry-mixed by a technique similar to the method described in Quantitative Analysis by G. Mahin (Lith ed., McGraw-Hill Book Co., Inc., N. Y., 1932). In such case, aliquots in amounts between 10 and 35 grams, preferably about 25 grams, are used with the medium ineach reactor; This method doesV not provide as good results as does the aforesaid wet-mix in the mixer prior to charging the reactor. It has been found better to use a portion of the entire mixture asV an inoculum rather than a portion of the supernatant liquid which might be termed soil extract.

It is to be understood that the sizesof the soil samples as well as the aliquots thereof charged tothe reactorsmay Vary dependingupon the particular experimental conditions employed.

rfowever, up to theA present time, samples and aliquots of the approximate sizes indicated above have been found most practical. and'productive of the best results.

The, subsequent procedure which follows the mixing isr for the purpose of determining the relative activities of the different soil. samples. This is determined by placing each soil-medium sample, in itsv reactor in contact with the. pr.e. selected mixture of` gases. including a hydrocar- Eil."

fThe curveV rises bon and continuing the contact under incubating conditions until the reaction iscompleted. While methane is preferred as the hydrocarbon, higher gases such as ethane, propane, ibutane, etc. may be used. The' time, usually measured in days, necessary toV reach this phase is an indication of the relative activity of the sample.

Referring to Fig. 4 which shows a curve plotted with time in days against the logarithim of the -milliliters of overall gas volume change, the

curve represents the decrease in overall gas volume in contact with the sample at given intervals of time. It will be noted first that the gas` change is rather low in the initial phase. relatively sharply through straight line portion of maximum activity un til the completed phase is reached. The latter is caused by the exhaustion of the oxygen or hydrocarbon contentv of the gas mixture, depending upon the initial composition of the ture. For convenience, theA elapsed time necessary to reach the beginning of the completed phase can be used to rate the several samples as to, activity.

Referring more specifically to the operations after the samples havebeen prepared and placed in the reactors, reference is made to -ligs. 1, 2 and 3 showing a reactor l lV and a so-called reservoir i2 which for convenience are glass prescription bottles of about 250 ml. volume, the bottles being accurately calibrated as to the 50 ml. mark. Reactor Il is closed by a rubber stopper having connections E3 and ifi, the connection lf3 extending below the 50 ml. mark.

., Reservoir'bottle l2 is closed with a rubber stopper having connections i5 and it, connection I5 extending down to the bottom of the bottle.

In filling a reactor, the reactor is rst filled with the desired gas mixture preferably composed of CO2, O2, or air and CHi and at substantially atmospheric pressure for ease in handling. The exact composition of this mixture can be varied widely and is so selected that either the oxygen or the methane, preferably oxygen, will be exhausted rst, thus indicating the be ginning of the completed phase shown in Fig. fi. Preferably, the` mixture is such that it is not explosive. A suitable mixture and one in which the oxygen is first exhausted is composed of 35% O2, 6G% Cl-Ii and 5% C02. The CO2 present primarily as a safety factor and its proportion can be varied to an extent such that it will not interfere with the reaction. rEhe O2 and CH4 content can be varied within wide limits, compatible with safety.

With the reactor filled with the gas mixture, preferably under a slight positive pressure to insure against contamination, the release of pressure being indicative of a properly stoppered bottle, connection ifi is connected as part of' a Siphon to the mixture in the mixer, the latter being kept operating at a rate such that no separation of thesoil-medium mixture therein tal-zesv place. An aliquot of the soil-medium sample is then siphoned into reactor H through connec tion le until 50 ml. of the sample is transferred tothe reactor. It is. desirable that the Siphoning bev continuous and that the solid particles in the sample be small enough so that there is noV blocking of the Siphon. tube. When this operation is completedV and connection it again closed, the reactor appears as shown in Fig. 2. Connection l3is then closed and connection is connected to connection i5 of reservoir i2 which contains more of the same medium as originally mixed with the soil sample in the mixer. This As already indicated, al1 the samples may be arrangement is shown in Fig. 3. carried to a completion of their reactions to de- With connection I 3 closed and connection IE termine the number of days required in each case of reservoir I2 open, the reactor contents are and the relative activities of the samples. Howincubated at a temperature of about 25 to 35 5 ever, it has been found as accurate and more ex- C., preferably about 28 to 30 C. After standing pedient to read the gas consumption 0f all Samfor some time, it will be observed that some ples on the day that the lrSt Samples complete of the medium has passed from reservoir I2 into their activity, e., attain the completed phase. reactor I I by reason of the consumption of gas In some cases as where one or two samples comin reactor Il by microorganisms in the soil plete their aCtiVity on the Same day, it may be sample. This transfer of medium is permitted advisable to wait one InOre day t0 determine to continue until a liquid level is reached in Whether the 'lrst Samples Were freaks. If on the reactor II indicative of a completion of the resecond day a larger number 0f Samples complete action or a beginning of the completed phase their reaction, the Second day Can then he taken wherein either the oxygen or the methane has as the irst day. Then the gas Consumption 0f been exhausted. After incubation, the nal pH all the samples regardless 0f their Stage of reachas been found to be in the Ordes of about 6,5 tion can be measured and plotted on a chart such to 7,0, as that shown in Fig. 5.

In a reactor of the type shown herein, a liquid While the data thus obtained can be related level of approximately 10o ml. has been found 2o directly to the area beine prospected. it is prefto indicate the desired completion of the reaction erahle to Subject the data irst `t0 a statistical as accurate as is considered necessary. Hence, aKalysis l'lo smooth it Out and Obtain a better it has been found most advantageous to dis- Showing of the Prospects for petroleum and gas. continue the incubation at that point and meas- Referring t0 Fig. 5 Which illustrates the gas ure the elapsed days which indicate the relative level Change in millimeters 1 mm. equals 2.2 m1.) activity of the Sampe. In the Case of a, more from Samples taken from the mark t0 about active sample, the number of days required to the 70 mark on the P line of Fig- '7, the actual reach thg 100 mark Oy the completed phase Values are Shown ln Solid lines. It be noted is much less than lthe number of days required that a saw-tooth effect is obtained which is to by a relatively meri; gamma be considered normal when determining a bio- It is to be understood that in the present case, logical eetVt- Nevertheless, it is evident from the attainment ef e liquid. level ef loo ml. has the profile that varying degrees of activity een been selected arbitrarily as a matter of conbe detected In the present eXample, all soil venience. However, it has been found close Samples found t0 produce Over a 20.0 mm. (44.0 enough for the purposes of the present invention ml-l decrease in test gas volume were considand has provided accurate and reproducible reered abnormally active. It is t0 be understood sults. In other types of reactors with different 'that in eel-oh Situation, this Value Will be selected types of medium and with dierent types of gas, arbitrarily and can be easily determined by inthis level may obviously vary. spectlon and comparison of the various values The residual gas mixture in the reactor is 40 obtained then analyzed as by an infra-red spectrophotoml In order to average the variations noted which eter to determine the actual amount of hydroare probably due to natural and experimental carbon consumed in milliliters. If the gas com- Causes e Convenient statistical method was position was originally so selected that oxygen adopted Such as a 3total, 3total, moving average was the gas to be completely consumed, the line- The Constrllotion 0f such a 3-total, 3-total actual amount of oxygen consumed would be moving average is graphically illustrated below: determined. This measurement is not essential 3total, 3total, moving average! Sample values A l? ll) E F G -rotal A+B+c e+c+l c+Dl+F J D+/E+r E+1t+e 3-total, S-total to an indication of the relative activity of the From the above, it will be seen that the stasarnple but is desirable to establish the normalcy tion to which the mean value is plotted is given of the reaction in the reactor and serves as a three times the weight of any other point, the two check upon the methane or oxygen consumpadjacent stations twice the Weight, etc. In tion. Thus, any freak or abnormal samples that progressing across the Station line, one station is are not truly indicative of the area being prosdropped, two carried, and one added as shown pected can be eliminated. above.

In the analysis, the slope of the straight line A 3-t0tal, 3total moving average was selected portions of the activity curves (Fig. 4) of the in this particular case because it introduced the most active samples, that is, those reaching the least distortion of the actual data as well as evibeginning of the completed phase in the least dent from the broken line in Fig. 5. For instance, number of days, constitute abnormally active a 5-tota1, 5t0tal, moving average was found to samples. Samples which are slow in incubation flatten the Weighted aVerage line and was, thereand in which the straight line portion departs fore,n0t desirable. materially in slope from the slope of the more As an illustration of an example of practicing,- active samples have been found to be normal. the invention, reference is made to the following The most active samples as represented by descriptions of actual field operation illustrated those samples having substantially parallel in Figs. 6 and 7. straight line portions are then plotted on a dia- Referring t0 Fig. 6, test stations in a selected gram of the area being prospe-:ted as will be test area were staked out as indicated by the grid explained later in connection with Figs. 6 and 7. 75 the area in question including grazing land, ari

orchard, a vineyard, a grain iield and a potato field. Some 281 composite soil samples were collected among approximately thirteen miles of test lines, the samples being prepared and incubated and the volume changes, rate of volume change and gas analyses being conducted as already described.

The results, after the aforesaid statistical analysis was applied thereto, are shown in Fig. 6, a simple dot indicating normal activity of the sample, a single circle indicating' a gas volume change of 44 to 55 ml., and a double circle indicating a gas volume change inexcess of 55 ml. Anything below a gas volume change of 44 ml. was considered to represent normal activity in this area.

In this survey the distance between stations was about 250 feet. As is evident from Fig. 6, abnormally active methane consuming areas tended toward grouping and gave some indication as to the location of the more active areas.

As a check upon the survey shown in Fig. 6, and also as a check upon the reproducibility of the data, a second and more detailed survey of the same area was made about iive weeks later during which some 352 samples along about 16 miles of test lines at 1A; mile intervals between test lines were collected.

The results of this survey are shown in Fig. 7. It will be noted that the active areas indicated in Fig. 6 are again indicated active in Fig. 7 and that the extent of such areas has been more closely determined. In every instance, there was confirmation of the original findings despite the elapsed time interval and despite the fact that some of this land had been subject to cultivation.

It is interesting to note that in the sampling of another area wherein there was a known fault through which gases were escaping to the surface from an oil pool beneath, the data secured indicated a hitherto unknown extension of the fault.

From the foregoing, it is believed evident that the present invention provides a relatively economical and accurate method of prospecting which the presence and relative amounts of gases in the earths surface indicative of petroleum, gas and other Subterranean deposits can be readily determined.

While the invention has been described in connection with hydrocarbon gas in the earths surface, it is to be understood that it is applicable to any type iluid capable of consumption by microorganisms.

I claim:

1. A method for prospecting for subterranean petroleum deposits characterized by the discharge of a hydrocarbon gas in the earth surface comprising the steps of collecting soil samples in the area being prospected, contacting aliquots of said soil samples under incubating conditions with a gas mixture comprising a gaseous aliphatic hydrocarbon and oxygen, said gas mixture being capable of utilization by hydrocarbon-consuming microorganisms whose presence is indicative of sub-surface petroleum deposits, eiecting said contact in the presence of a substantial amount of added inorganic salt nutrient medium, said medium being selected so as to encourage optimum growth of said hydrocarbon-consuming microorganisms and employed in an amount suiicient to eliminate variables of moisture and nutrient content of said soil samples and to maintain the pH of said incubating mixture at a level conducive to optimum growth of said hydrocarbon-consuming microorganisms, determining the relative rates of hydrocarbon gas consumption in said gas mixture by said soil samples under said growth conditions and correlating said rates with the area being prospected.

2. A method according to claim 1 wherein said gas mixture comprises mainly methane and oxygen.

3. A method according to claim 1 wherein said gas mixture comprises about 35 per cent oxygen, per cent methane and 5 per cent carbon dioxide.

4. A method according to claim 1 wherein approximately 7 Inl. of said nutrient medium is added for every 3 to 4 grams of soil sample.

5. A process according to claim 1 in which said soil sample is mixed with said nutrient medium under conditions of agitation and a portion of the soil-nutrient mixture is withdrawn for contact with said gas mixture while said mixture is agitated suiiiciently to prevent any substantial separation of substituents.

6. A process according to claim 1 in which said soil samples are maintained at a temperature of about 42 F. from the time they are collected until an aliquot thereof is contacted with said gas mixture.

RAYMOND J. STRAWINSKI.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 2,294,425 Sanderson Sept. 1, 1942 2,349,472 Taggart May 23, 1944 OTHER REFERENCES Porter, Bacterial Chemistry and Physiology, 1947, pages 660-661. 

1. A METHOD FOR PROSPECTING FOR SUBTERRANEAN PETROLEUM DEPOSITS CHARACTERIZED BY THE DISCHARGE OF A HYDROCARBON GAS IN THE EARTH SURFACE COMPRISING THE STEPS OF COLLECTING SOIL SAMPLES IN THE AREA BEING PROSPECTED, CONTACTING ALIQUOTS OF SAID SOIL SAMPLES UNDER INCUBATING CONDITIONS WITH A GAS MIXTURE COMPRISING A GASEOUS ALIPHATIC HYDROCARBON AND OXYGEN, SAID GAS MIXTURE BEING CAPABLE OF UTILIZATION BY HYDROCARBON-CONSUMING MICROORGANISMS WHOSE PRESENCE IS INDICATIVE OF SUB-SURFACE PETROLEUM DEPOSITS, EFFECTING SAID CONTACT IN THE PRESENCE OF A SUBSTANTIAL AMOUNT OF ADDED INORGANIC SALT NUTRIENT MEDIUM, SAID MEDIUM BEING SELECTED SO AS TO ENCOURAGE OPTIMUM GROWTH OF SAID HYDROCARBON-CONSUMING MICROORGANISMS AND EMPLOYED IN AN AMOUNT SUFFICIENT TO ELIMINATE VARIABLE OF MOISTURE AND NUTRIENT CONTENT OF SAID SOIL SAMPLES AND TO MAINTAIN THE PH OF SAID INCUBATING MIXTURE AT A LEVEL CONDUCTIVE TO OPTIMUM GROWTH OF SAID HYDROCARBON-CONSUMING MICROORGANISMS, DETERMING THE RELATIVE RATES OF HYDROCARBON GAS CONSUMPTION IN SAID GAS MIXTURE BY SAID SOIL SAMPLES UNDER SAID GROWTH CONDITIONS AND CORRELATING SAID RATES WITH THE AREA BEING PROSPECTED. 